Vulcan is a hypothetical intra-Mercurian planet, the existence of which was proposed by the 19th century French mathematician Urbain Leverrier to explain anomalies in Mercury's orbit (now accounted for by the general theory of relativity). Alleged sightings of Vulcan in 1859 inspired many astronomers to look for it during the total solar eclipse of 1860. Nothing was found. In recent years, however, a number of exoplanets have been discovered with orbits much smaller than that of Mercury. These so-called epistellar jovians, also known as 51-Pegasi-type planets or "hot Jupiters," have forced scientists to reconsider their models of planetary system formation. See also vulcanoid.

Vulcan is also the homeworld of the Vulcan people in the Star Trek universe. Vulcan, a hot, dry planet, is supposed to lie in the 40 Eridani star system, 16 light-years from Earth. Star Trek creator Gene Roddenberry endorsed 40 Eridani as Vulcan's sun in a letter published in the July 1991 issue of Sky & Telescope magazine.